Three aircraft which had been sent to the search area on Wednesday were recalled when heavy seas and poor visibility increased.

"Current weather conditions are resulting in heavy seas and poor visibility ... making air search activities ineffective and potentially hazardous," the Joint Agency Coordination Centre (JACC) said in a statement.

What is the Bluefin-21?

An autonomous underwater vehicle (AUV) designed for deep-sea surveying.

It has a "swappable payload". It will first use sonar in the search and will be refitted with cameras if something is detected.

It's 5m long and weighs 750kg. Has an endurance of 25 hours underwater at a speed of 3 knots, with a top speed of 4 knots.

It has a depth rating of 4,500m, meaning it will be at its limit in the Indian Ocean search zone.

Bluefin Robotics says its AUV can also be used for archaeology, oceanography, mine countermeasures, and unexploded ordnance.

The twelve ships involved in today's search will continue the hunt for wreckage from the Boeing 777, which vanished on March 8 carrying 239 people, including six Australians.

Australia has vowed to keep searching for the missing plane as autonomous underwater vehicle Bluefin-21 nears the end of its first full mission.

"Bluefin-21 has now completed more than 80 per cent of the focused underwater search ... no contacts of interest have been found to date," the JACC statement said.

Search officials have said that once the Bluefin-21's current mission, 2,000 kilometres north-west of Perth, is finished, they will redeploy the submarine to other areas yet to be determined.

Prime Minister Tony Abbott says the search strategy may change if seabed scans from the drone fail to turn up any signs of debris.

"We may well re-think the search but we will not rest until we have done everything we can to solve this mystery," he said.

"The only way we can get to the bottom of this is to keep searching the probable impact zone until we find something or until we have searched it as thoroughly as human ingenuity allows at this time."

Bluefin-21, a key component in the search after the detection of audio signals or "pings" believed to be from the plane's flight recorder, is due to end its first full mission soon.

The Australian and Malaysian governments are under increasing pressure to show what lengths they are prepared to go to in order to give closure to the grieving families of those on board.

In a sign of the families' growing desperation for answers, a group purporting to be relatives of the missing flight's passengers published a letter to Malaysian defence minister Hishammuddin Hussein, urging the government to investigate old media reports that the plane landed in Kandahar, Afghanistan.

"It is high time that the government should start thinking out of the box by exploring and re-examining all leads, new and old," said the letter which was published on Facebook.